According to the current technology, the material generally used as a substrate for forming gallium-nitride based light-emitting diodes (LEDs) is sapphire (Al2O3) or silicon carbide (SiC). When a GaN semiconductor layer is grown on the substrate, lattice defects or dislocations may occur in the semiconductor layer because the lattice constant or the crystal structure of the substrate are different from the semiconductor layer. The defects may extend from the bottom of the semiconductor layer to the surface of the LED, damage the light-emitting layer of the LED and reduce the light-emitting efficiency of the LED.
So that, one of prior methods for improving the light-emitting efficiency of the LED is to reduce or decrease quantity or density of the defects. According to the current technology, as disclosed by U.S. Pat. No. 7,521,274 ('274 patent), one kind of approaches to reduce the defects is forming a GaN rod structure in nanometer scale on a substrate before forming a GaN semiconductor layer thereon. As shown in FIGS. 1A to 1C, a patterned mask 13 which having a plurality of holes in nanometer scale for exposing a partial surface of a substrate 7 is disposed on the substrate 7. Then a semiconductor material 81 is directly grown in the holes by using non-pulse method, and forming a plane with the mask 13. After that, the semiconductor rods 82 are subsequently grown on the semiconductor material 81 by using pulse method.
There is another kind of approaches to grow GaN nano-rods on a silicon substrate as disclosed by U.S. Part. No. 7,981,714 ('714 patent). As disclosed in the '714 patent, the first step is to perpendicularly grow a plurality of nano-rods on a silicon substrate directly. The second step is to fill the gaps among nano-rods with an amorphous material while expose upper portions of the nano-rods, which is quite difficult. Finally, the third step is to use part of the upper portions of the nano-rods as seeds of crystallization to grow GaN on the amorphous material.